The cable which ties the Stereo into the IQ-2020 in your spa is a custom-made device which only works for the particular CD player which comes with the Spaudio II. I guess you could find out exactly which model it is, then get the part number for the interconnect, and then put the stereo somewhere nearby - on a AC to DC 12 volt converter, and hope for the best...
But I sure wouldn't want to figure out who pays for what if you have any warranty issues at that point.